Three-phase electrical distribution networks, for example distributing an electrical power supply through a building, are subject to harmonic currents generated by single-phase non-linear loads such as electronic equipment and equipment that uses different kinds of arc processes. Such equipment can generate excessive harmonic currents in the distribution network, including zero phase sequence (current) harmonics. Zero phase sequence harmonics, together with unbalanced portions of the fundamental and other harmonic currents, are additive in the neutral conductor, which can result in cumulative currents well in excess of the anticipated phase currents and overload the neutral conductor, which is not protected. In addition to the possibility of overload, these harmonics result in high common-mode noise level (neutral to ground voltage), increased total harmonic distortion level, voltage imbalance, increased power losses and other problems which are well known.
In a three-phase distribution network, zero phase sequence harmonics are conventionally controlled using zero phase sequence filters. Such filters have a low impedance to zero phase sequence harmonic currents, and as such attract these currents and effectively reduce their flow in the distribution network. The most common zero phase sequence filter is conventionally known as a "zig-zag" reactor or autotransformer.
In general, a zero phase sequence filter will be designed to offer the lowest possible impedance to zero phase sequence harmonic currents, but there are situations in which a higher impedance is needed to meet specific system requirements. Conventionally, a zig-zag filter will have to be custom designed for situations where a higher impedance is required, and in the case of an existing distribution network the existing filters must be replaced to accommodate the specified higher impedance. This is particularly troublesome in present times, where for example in commercial buildings expansion of facilities or changing tenants can give rise to new system requirements relatively frequently.
There are a number of methods currently available for adjusting the impedance of a zig-zag filter, but these are complicated and expensive, requiring sophisticated electronic equipment, and are relatively labour intensive.
The present invention overcomes these disadvantages by providing a method of adjusting the impedance of a zero phase sequence harmonic current filter and a field-adjustable zero phase sequence filter utilizing the method of the invention. The adjustment or "detuning" of the filter is easily accomplished in situ by maintenance personnel, without the need for extraneous equipment, and the zero phase sequence impedance of the filter can thus be changed as needed to accommodate changing demands on an electrical distribution network.
This is accomplished in a preferred embodiment by providing a zig-zag filter with one or more taps on one of the windings of each core leg, or an auxiliary winding connected to one of the windings on each core leg, which allow maintenance personnel to alter the effective size of the winding and thus the zero phase sequence flux produced by the winding.
In a preferred embodiment this change can be made independently for each core leg. The zero phase sequence impedance of a particular phase increases as the windings on the core leg for that phase are unbalanced or "detuned", and the increase in impedance is cumulative as between the three phases. Therefore, by providing a single tap or auxiliary winding associated with one winding for each phase, four steps of zero phase sequence impedance are made available.
The taps or auxiliary windings can be accessed as required, either by conventional switching devices or by physically unplugging the connection to the winding input terminal and reestablishing the connection through the tap or auxiliary winding input terminal.